I think were they shoot themselves in foot with Lost is the whole time continuum requirements. As in, in the middle of 3 year of series they are what, second or third month on the island. You can only keep actors looking the same for so long. For the whole "this stuff happens every day" storyline they can't gain weight, get different shortcut, can't be casted in different movies that require styling, so on so forth. Four series will be quite enough, although I smell publicity stunt for the come back - as in they will say network has issued end date, people will riot, fans will sign petitions, press will blow it out of proportions, just what's needed for a big come back after massive break in the middle of the series (wasn't that stupid move by the network btw?)
As for 24 - this is excellent example of the series that outstayed its welcome. The plot is already moved like 5 years ahead from now, borderline ridiculous, and judging by the ..ahem... reviews of the new season DVD preview thingy.. they completely lost time continuum aspect of 24 hours. In new series people alledgedely drive from LAX to city centre in one commercial break, and the infamous never sleeping, never drinking, never eating, pooping or peeing agents perform all kinds of hyperspace stunts all across the state in matter of 3 hours, which is apparently just enough to move 3 blocks in LA in morning rush hour.